1. Preheat oven to 350°F/180°C. Grease and line an 8″ (20cm) square pan.
2. In a bowl, combine dark chocolate, cocoa powder, espresso powder and sea salt. Pour boiling water over and whisk until you achieve a smooth chocolate mixture.
3. Add coconut oil, tahini, sugar and vanilla and whisk to combine. Leave to cool for 5 minutes.
4. Once cooled, add flour and baking powder to the mixture and whisk till smooth. Fold in dark chocolate chunks and walnuts.
5. Pour batter into pan, sprinkle with sea salt and bake for 20 minutes. Remove from oven and leave to cool for at least 2 hours or until completely cool before cutting (you can also chill them in the fridge).